Conventionally, when an image is formed using a decolorable toner, a decolorable toner of a single color is used. If an image is formed using a plurality of decolorable color toners, a problem occurs. When a fixing temperature of one of the plurality of decolorable color toners is higher than a decoloring temperature of another of the plurality of decolorable color toners and all of the decolorable color toners are fixed on a sheet, a part of the image formed of decolorable color toners is decolored.